During the past two years we have attempted to localize adenylate cyclase and phosphodiesterase in Euglena. Since fractionation methods for Euglena are crude, our efforts have only been partially successful. Presently we have a crude pellicle preparation enriched in phosphodiesterase activity and a flagellar fraction containing high adenylate cyclase activity. We plan to further purify each of these fractions and to characterize the associated enzyme. We have evidence that light influences the activities of both adenylate cyclase and phosphodiesterase, the former increases at three hours of greening, while the latter decreases. Fixed carbon sources, known to inhibit chloroplast development, are currently being tested to ascertain their effect on these enzymes in both light- and dark-grown cells. Nitrogen sources, known to relieve the inhibition of chloroplast development caused by certain fixed carbon sources, are also being studied in regard to their effect on the enzymes associated with cAMP metabolism.